Reproductive Success in Dairy Cows: the Linkage of Milk Yield, Nutrition, and Reproduction

During the past several decades, selection for increased milk yield has had a temporal association with declines in fertility and reproductive efficiency in dairy cows. In the US, an average annual decrease of 0.5% in conception rate at first service occurred between 1975 and 1997 (Butler and Smith, 1989; Beam and Butler, 1999). In the UK (between 1978 and 1996) and in Spain (between 1991 and 2000) conception rate at first service decreased 1% per year (Royal et al., 2000; Lopez-Gatius, 2003). Because decreased fertility is one of the primary reasons to cull cows and represents a major economic loss to dairy producers ($0.3 billion/year in the USA), this association has generated considerable interest and concern in the dairy industry. Although the temporal association indicates a significant problem, it does not imply a direct effect of milk yield on reproduction. An examination of the underlying biology is needed to ascertain the reason(s) for this association. Genetics, environment (management), and their interaction clearly contribute to these concurrent trends for increased milk yield and decreased fertility. However, despite considerable effort, the specific physiological mechanism(s) responsible for the linkage of these trends remains unknown. It is apparent that management plays a major role and efforts to increase cow comfort and to minimize stress are beneficial, if not crucial, for successful lactation and reproduction performance. The goal of this presentation is to provide a brief overview of genetic and environmental effects on reproductive physiology and to describe how reproductive, metabolic, and stress-related hormones interact to affect reproductive performance of the contemporary cow.